Safety device for ordnance fuzes



Sept. 9, 1958 P. J. FRANKLIN 2,850,978

SAFETY DEVICE FOR ORDNANCE FUZES FiledMarch 2, 1955 LOW- MELTI NG ALLOYFUZE CIRCUITRY SETBACK DEVICE INVENTOR Ph/Y/p J. Frank/in I ATTORNEYSSAFETY DEVICE FOR ORDNANCE FUZES Philip J. Franklin, Washington, D. C.

Application March 2, 1955, Serial No. 491,809

8 Claims. (Cl. 102-701) (Granted under Title 35, U. S. Code (1952), sec.266) The invention described herein may be manufactured and used by orfor the Government for governmental purposes without the payment to meof any royalty thereon.

" This invention relates to fuzes for ordnance missiles and tosafety-and-arming mechanisms therefor.

Typical ordnance fuzes are designed to change from an insensitivecondition, known as the safe or unarmed condition, to a sensitive orarmed condition in response to setback-4. e., in response to forces dueto the acceleration applied to the missile at the beginning of itsflight. The fuze is adapted, when in the armed condition, to causeexplosion of the missiles warhead when the missile encounters certaininfluences such as contact with or proximity to a target. When the fuzeis in the unarmed condition, however, it is insensitive to these sameinfluences.

It is essential to the safety of using personnel that an ordnance fuzeremain in the unarmed condition until the missile in which it is mountedhas been propelled toward its target and is a safe distance away fromusing personnel. In particular, the fuze must not accidentally arm as aresult of rough handling or accidental dropping. Setback-responsivesafety-and-arming mechanisms for fuzes are very carefully designed tominimize the possibility of such premature arming. Nevertheless, thepossibility does exist of accidental arming, as a result of mechanicalfailure or extraordinary combinations of forces or both. The newertactics of airborne warfare present a particular problem, calling asthey do for ammunition that can be dropped from the air to using troops;such ammunition may be subjected to very severe forces on landing,particularly in the event of failure of parachutes or otheracceleration-reducing devices. The accidental arming of even a very fewsetback-arming fuzes may be quite intolerable.

The ordnance fuzing art has increasingly felt the need for safetyand-arming devices insensitive to setback devices that would remain saferegardless of forces due to acceleration but that would arm, in flight,at a safe dstance from using personnel, in response to some effect notrelated to setback. Such setback-insensitive safetyand-arming devicescould be used alone but would preferably be used in combination withsetback-responsive or other arming devices; with such a combinationarrangement the fuze would not become fully armed until both thesetback-responsive device and the setback-insensitive device hadindependently changed from the unarmed to the armed condition. Such asetback-insensitive arming device, if reliable and practical, could makepractical the fuzing ofordnance missiles with a degree of safetyhitherto unknown. The principal object of my invention is to providesuch setback-insensitive safety-and-arming devices and methods forordnance fuzes.

Briefly, my invention provides safety-and-arming devices in which heatdeveloped by air friction against the body of the missile is utilized tocause a change from the safe to the armed condition. For use incombination with well-known safety-and-arming devices havingdetonator-carrying rotors adapted to rotate from an unarmed to an armedposition, I prefer to provide a mechanical element constrained to afirst position by a lowmelting solid mounted inside the missile close tothe periphery of the missiles nose. When in this position the element isadapted to prevent rotation of the rotor to the armed position. When thenose of the missile reaches a temperature to be expected in normalflight, the solid melts and the element moves automatically to a secondposition in which it no longer prevents rotation of the rotor to thearmed position. If the fuze is of an electrical type, a desirable formof my invention is one in which the same low-melting solid iselectrically conductive and short-circuits an electric detonator, orother critical circuit component; the detonator cannot function untilthe alloy has melted and removed the short circuit.

Other aspects, objects, uses, and advantages of my invention will becomeapparent from the following description and from the accompanyingdrawing, in which:

The drawing is a longitudinal section, not to scale, showing principalfeatures of an ordnance missile incorporating my invention.

In the drawing, numeral 12 designates the ogive or outer shell of thenose of an ordnance missile. The missile is provided with a fuze havinga safety-and-arming mechanism that includes an arming rotor 14, whichmay be of a well-known type. Rotor 14, shown in the unarmed position,rotates on shaft 16 and is adapted to arm the fuze upon rotation to anarmed position. Rotor 14 is loaded by a spring (not shown) that urges itinto the armed position. Until actuated by acceleration forcescorresponding to those produced on normal setback, however, asetback-responsive device 18 holds rotor 14 in the unarmed position asshown. When rotor 14 is in the armed position, an electric detonator 40is adapted to detonate upon receiving an electrical voltage fromappropriate fuze circuitry 42. Fuze circuitry 42 is adapted to provide adetonating voltage upon receiving an appropriate indication of thepresence of a target, in accordance with well-known practice. Theelements thus far described are well known and their operation will bewell understood, without more detailed description, by persons skilledin ordnance fuzing.

In the embodiment of my invention shown in the drawing, I have provideda hole 20 through rotor 14, the axis of hole 20 being generallyperpendicular to the axis of rotor 14. A wire 22 passes through aprotective tube 23 and through the hole 20. End 26 of wire 22 is held bya low-melting alloy 24 that constitutes the'forward tip of the missile.The other end 27 of wire 22 is held under spring tension by aspring-loading device 25. Besides holding wire 22, alloy 24 electricallygrounds an electrode 31 to tube 23 and/or to ogive 12. Electrode 31 iselectrically connected to one terminal of an electrical detonator 40,the other terminal of which is grounded to tube 23. Detonator 49 is thusshort-circuited by alloy 24.

Even though mechanical setback device 18 may have been actuated and mayno longer impede the rotation of rotor 14 to the armed position, rotor14 is prevented from so rotating as long as wire 22 passes through hole20 and is held under tension. Furthermore, as long as alloy 24short-circuits electrode 31 to tube 23, detonator 40 will not detonate,even though a detonating voltage be prematurely generated by fuzecircuitry 42.

When low-melting alloy 24 is raised to a temperature corresponding tothat resulting from friction of air thereag'ainst in normal flight,however, alloy 24 melts, unshorting detonator 40 and freeing end 26 ofwire 22.

Patented Sept. 9, 1958 Spring-loading device 25 now withdraws wire 22from hole 20, so that wire 22 no longer prevents rotor 14 from rotatingto the armed position. Provided setbackresponsive device 18 has beenactuated, the missile will now be fully armed.

It will be understood that'spring-loading device 25 maytake any of anumber of 'well-known forms adapted to maintain tension-on wire 22 priorto release of end 26 and to thereafter withdraw wire 22 fromhole 29. Acoiled spring of the type used in ordinary clocks and watches is onesuch form.

I prefer to select for wire 22 a material having a slight stiffness, thestiffness being suflicient to resist rotation of spring-loaded rotor 14to the armed position until wire 22 is fully withdrawn from hole 20,even though end 26 may have been'released by the melting of alloy'24.Alternatively, however, a material may be selected'for wire 22 having astiffness so low that, as soon as the tension of wire 22 is releasedupon the melting of alloy 24, rotation of spring-loaded rotor 14 to thearmed position is not resisted.

It will be apparent that various alternative ways of releasing a lock onrotor 14 upon the melting of alloy24 can be provided, One advantage ofthe wire arrangement I have described, however, is that it is readilyadapted to missiles of existing designs in which the fuze rotor islocated a considerable distance rearward of the forward extremity of themissile. It is in general readily feasible to modify the design of sucha missile to provide for wire 22 to run from the forward extremity torotor 14.

In the embodiment shown in the drawing, part of alloy 24, upon melting,may be forced into protective tube 23 by air pressure. Tube 23 may beadapted to protect critical parts of the fuze from unwanted effects thatthe alloy particles might otherwise cause.

It will be understood that the drawing is not to scale and that alloy 24preferably comprises a relatively small part of the nose of the missilein relation to the overall size of the missile. Modification of theaerodynamic characteristics of the missile caused by the melting ofalloy 24 will thus be minimized and can be made negligible.

It will also be understood that the outer surface of alloy 24 need notbe directly exposed to the atmosphere; I

allow 24 can, if desired, be separated from the atmosphere by a layer ofhigh-melting material. Such a layer should preferably be thin, have highthermal conductivity, and make good thermal contact with alloy 24.Furthermore, to facilitate flow of alloy 24 upon melting, such a layershould preferably be vented.

Although I have shown an embodiment of my invention in which alloy 24fusibly short-circuits an electric detonator 40, it will be understoodthat, alternatively, other critical circuit points may be fusiblyshort-circuited when my invention is used in association with fuzes ofelectrical types.

Manifestly, the principal utility of my invention is in its applicationto missiles having normal velocites sufficent to cause substantialtemperature rises as a result of air friction. Many if not most of themissiles now known and used by the military do fortunately have suchvelocities, and a continuing trend to higher velocities and higherresulting missile temperatures may be expected. Some missiles that havebeen used by the military for years quickly develop skin temperatures ofthe order of 400 or 500 degrees centigrade in flight. Such temperaturesare obviously well above those to which missiles are subjected in normalshipping, storage, and handling prior to use. Furthermore, for purposesof my invention, slight modifications of the surface of a missile in theregion of its forward extremity may be found to increase the temperaturerise in such region. Alloys and other materials having suitable meltingpoints and other properties for use in my invention are well known andreadily available. It will thus be clear that my invention, besidesbeing readily practicable at low cost in the light of the foregoingdisclosure, has great utility and wide applicability.

It will be apparent that the embodiments shown are only exemplary andthat various modifications can be made in construction and arrangementwithin the scope of the invention as defined in the appended claims.

I claim:

1. In an ordnance missile, in combination with an arming mechanismhaving safe and armed positions, means for preventing the arming of saidfuze until a portion of the body of said missile reaches a predeterminedtemperature, said means comprising: a solid material mounted in the noseportion of said missile, said solid material being'adapted to melt whenthe nose of the missile attains a temperature to be expected as a resultof air friction in normal flight; and a mechanical element held in afirst position by said solid material and adapted to move automaticallyto a second position upon the melting of said solid material, saidelement constraining said mechanism to said safe position when in saidfirst position and permitting said mechanism to move to said armedposition when in said second position.

2. In an ordnance missile, in combination with. an ordnance fuze havinga mechanical rotor, said fuze being adapted to become armed upon therotation of said rotor from an unarmed to an armed position, safetymeans for preventing the arming of said fuze until a portion of the bodyof said missile reaches a predetermined temperature, said meanscomprising: a solid material mounted in the nose portion of saidmissile, said solid material being adapted to melt when the nose of themissile attains a temperature to be expected as a result of air frictionin normal flight; and a spring-loaded mechanical element held in a firstposition by said solid material and adapted to move to a second positionupon the melting of said solid material, said element when in said firstposition being adapted to mechanically prevent rotation of said rotorfrom said unarmed position to said armed position and said element whenin said second position being adapted to permit rotation of said rotorfromsaid unarmed position to said armed position.

3.; In an ordnance missile, in combination with an ordnance fuze havinga mechanical rotor, said fuze being adapted to become armed upon therotation of said rotor from an unarmed to an armed position, safetymeans for preventing the arming of said fuze until a portion of the bodyof said missile reaches a predetermined temperature, said meanscomprising: an alloy mounted in the nose portion of said missile, saidalloy being adapted to melt when the nose of the missile attains atemperature to be expected as a result of air friction in normal flight;said rotor having an aperture therethrough, the axis of said aperturebeing generally perpendicular to the axis of said rotor; a wire fusiblyheld at its first end by said alloy, said wire passing through saidaperture when said rotor is in the unarmed position; and spring meansfor holding the second end of said wire under tension, so that the wireprevents said rotor from rotating from the unarmed to the armed positionuntil said alloy melts and releases the first end of the wire.

4. The invention according to claim 3, said spring means being of thecoiled spring variety and bein adapted to withdraw said wire completelyfrom said aperture.

5. The invention according to claim 3 comprising additionally: firstandsecond circuit points, said fuze being adapted to detonate only if avoltage is applied to said circuit points; and first and secondelectrodes electrically connected to said first and second circuitpoints respectively, said first and second electrodes being fusiblyshort- .circuited by said alloy, said electrodes being adapted to becomeopen-eircuited upon said alloy reaching a temperature to be expected asa result of air friction in normal flight.

6. The invention according to claim 5 comprising additionally anelectric detonator connected across said first and second circuitpoints.

7. The invention according to claim 5 comprising additionally a voltagesource across said circuit points.

8. In an ordnance missile, in combination with an electrical ordnancefuze having first and second circuit points, the fuze being adapted todetonate only if a voltage is applied to said circuit points, safetymeans comprising: a fusible low-resistance electrical path effectivelyshort circuiting said circuit points, said path including at least afusible portion located in proximity to the surface of the nose portionof said missile, said fusible portion being adapted to melt when thenose portion of the missile reaches a temperature produced by airfriction during normal flight and thereby to unshort said circuitpoints.

References Cited in the file of this patent UNITED STATES PATENTS

